The allure of the Harley-Davidson 103″ Twin Cam was one of the factors leading me to purchase my 2012 FLD Switchback. However, for the 103″  to perform at its highest potential, fuel management and airflow needed to be improved. And all bikes come from the factory running lean, evidenced by the popping sound that occurs upon deceleration. So one of the fi rst modifi cations I planned was an upgrade to the exhaust, air cleaner and electronic fuel injection. I took my time in selecting a solution, and based on my research, the Zipper’s Performance ThunderMax product set seemed to fit the bill.

Bobby Goad does the baseline dyno runs on my Switchback prior to the install
Bobby Goad does the baseline dyno runs on my Switchback prior to the install

Zipper’s Performance was started in 1978 by Dan Fitzmaurice and Dave Zehner, with the two building and selling racing engines out of a garage. In 1985 John Kitzmiller joined them, and Zipper’s was transformed into a resource for performance-minded customers. The partners’ racing engine background prepared them for the development of performance engine products for both the street and the track. The company does its product manufacturing at their Elkridge, Maryland, location, which houses fully equipped machine and assembly rooms, dyno rooms and shipping facilities.
A decade ago Zipper’s teamed up with Thunder Heart, a performance company based in White House, Tennessee, to develop the ThunderMax product line. The ThunderMax with Auto Tuning Fuel Injection Performance Module (ThunderMax with AutoTune for short) is a self-tuning fuel management component that actually replaces the stock EFI engine control module. It employs wide-band oxygen sensors, meaning that instead of switching back and forth from rich to lean like narrow-band Harley-type sensors do, the ThunderMax oxygen sensors produce a signal that can measure the air/fuel ratio directly, responding to changes in the air/fuel mixture in milliseconds. The software contains hundreds of base maps that can apply to the majority of factory and aftermarket component combinations. Thunder Heart does the manufacturing and assembly, while both companies work together on software and map development, dyno testing and working with customers to do pre-release, real-world road testing. ThunderMax with AutoTune is backed by a three-year warranty, proving the company’s commitment to the product.
Justin Bender installs the Zipper's HiFlow Air Filter Kit
Justin Bender installs the Zipper’s HiFlow Air Filter Kit

Knowing that I would be attending Delmarva Bike Week in Ocean City, Maryland, last September, Rosie invited me to the Zipper’s Performance facility in Elkridge, Maryland, to have the products installed right after the rally. I jumped at the chance. What better place to have performance products implemented and my bike tuned than where the experts work?
Once those procedures had been completed, he removed my exhaust system. Detailed installation instructions are included with each product and are also available online, so I could follow along with each of the installation steps the crew performed. The stock Harley exhaust system supported 18mm ThunderMax oxygen sensors up until 2011 when Harley changed to 12mm bungs, which are not compatible with the 18mm sensors. Since I chose to keep the stock header pipes, some modifications had to be done. Zipper’s offers weld-in oxygen sensor bungs with caps (straight are $9.95 each and angled are $11.95 each). Dave Zehner, who is also a machinist and welder extraordinaire, removed the stock oxygen sensors and bungs from the pipes and plugged the bungholes. He then drilled new bungholes to accommodate the 18mm ThunderMax sensors in a new location, taking care not to let any debris fall into the pipes. The sensors must be placed within a few inches of the exhaust ports, pretty much where they’re located on 2006–2011 models. When the drilling and fitting was done, Dave welded the bungs in place. Note that if I’d chosen one of the aftermarket header pipes that Zipper’s offers, this entire step would not have been necessary. That said, Rosie provided me with a set of Drag Specialties O2 Sensor Adapters if I ever decide to revert back to the stock electronic control module (ECM).
Dave Zehner drills new bungholes in the Switchback exhaust for the new oxygen sensors
Dave Zehner drills new bungholes in the Switchback exhaust for the new oxygen sensors

Zipper’s Justin Bender removed the stock air cleaner and back plate from my bike and installed the Zipper’s HiFlow air filter kit. I chose the 2 1/4″ deep HiFlow instead of the 2 3/4″ deep MaxFlow because I wanted it to fi t behind my air cleaner cover without extending it outward. Those of you who are about 5′ 6″ or under will appreciate this option. Both the HiFlow and MaxFlow increase airflow dramatically, as the stock air filter is only about 1” deep. An adapter gasket ($16.95) is available that allows the retention of the factory teardrop cover used in ’08-up Dyna models. Also offered is a Zipper’s $99.95 chrome or chrome scalloped round cover. The HiFlow kit comes with a back plate and all necessary hardware and gaskets. The back plate is machined onsite, using high-quality aircraft billet aluminum, and is also available in black or chrome.
By the time the pipes came back from the machine shop, Bobby had already detached the battery cables on the bike and removed the stock ECM from the wiring harness. The ThunderMax with AutoTune ECM was plugged in and the new oxygen sensors connected. The next step was loading the map into the ECM using the TMaxII Tuner software. Bobby selected the appropriate map for my bike’s configuration, based on engine size, fuel injector size, camshaft, exhaust system and other elements. The map installation was done through a series of easy-to-understand pages in the Tuner software. Once the customization was completed, the base map was transferred into the ThunderMax ECM via a cable connecting the computer to the ECM. Then the initialization procedure, which involved tasks like cycling the ignition switch on and off several times, was done.
Because this was the first time Bobby had seen a Switchback exhaust, he put the bike on the dyno again for several reasons: to give the module a head start on its self-adjustment journey before my ride home, observe how the fuel points were tuning and verify that some pinging he’d heard on the initial runs was eliminated, and to get a new set of dyno run results. I was quite pleased to see that max horsepower increased from 64.98 to 73.64, and max torque from 86.89 to 90.12. Note that dyno runs are not necessary for tuning, as the module is truly self-adjusting. While the bike is ridden, the sensors provide near-constant feedback to the ECM, which adjusts itself almost simultaneously in response to conditions. Every startup is a new learning experience for the ECM.
Note the difference between the stock 12mm narrow-band oxygen sensors (left) and the ThunderMax 18mm wide-band sensors
Note the difference between the stock 12mm narrow-band oxygen sensors (left) and the ThunderMax 18mm wide-band sensors

By late afternoon, my bike and I were ready to hit the road. My ride home was over 200 miles, giving me plenty of time to observe the differences between the stock setup and the new ECM and air filter. The first thing I noticed when I got back on the highway was the smoothness of the throttle response—no jerkiness like I’d experienced before the install. I also noticed the sound of the air cleaner, which produced a nice, throaty warble when I cracked the throttle. (Instead of the stock back plate that closed off most of the air, the Zipper’s one leaves a space between the back plate and the air cleaner.) I also observed that even at revs less than 2000 rpm, I could get on the gas and the bike accelerated smoothly with no lugging. This is a double-edged sword: It’s not a good idea to get into the habit of lugging the bike, but it’s comforting to know that I can summon power even at a low rpm range. Where the ThunderMax really shines, though, is at the higher end of the rpm range. Now I know what a 103″ motor is supposed to feel like!
Justin had mentioned that it would take awhile for the bike’s low fuel indicator light to stabilize. Sure enough, the light came on every time I started the bike with the fuel level at a quarter tank or less. I also noticed that my gas mileage was slightly lower than the 43 mpg I usually got, but Bobby had informed me that my gas mileage would improve as the ThunderMax with AutoTune continued “learning” how to set the optimal fuel/air mixture.
The ThunderMax software shows the fuel flow adjustment that takes place as the bike runs
The ThunderMax software shows the fuel flow adjustment that takes place as the bike runs

A few weeks after the install, Dan asked me to take my bike to Sussex Hills Ltd. Dan wanted to get a readout from the ECM using the TMax Auto Support product, and owner Norm Gross is one of his top ThunderMax dealers. As Dan explained, “I like to hover over to make sure everything is working fi ne. This neat feature allows anyone to collect the learned data in the module to send in to our tech specialist. We use it to track how well the base maps are learning fuel points as riders incur large swings in ambient temperature, etc. And this info provides clarity to remedy almost any mechanical or electric problem on the motorcycle.” If you ever see Dan at a rally, you will notice that he spends much of his time at his computer, working with the ThunderMax software.
The ThunderMax with AutoTune is a plug-in replacement to the stock ECM
The ThunderMax with AutoTune is a plug-in replacement to the stock ECM

Norm’s shop in Sussex, New Jersey, is less than an hour from where I live, so I made plans to ride up there the following week. When I arrived, tech Matt Liptak connected the shop computer, already equipped with the TMax Auto Support software, to the ThunderMax ECM using the cable that came with the ThunderMax with AutoTune product. Through a series of interactive screens, Matt collected data about my bike. This is where I really learned about the Thunder Max with AutoTune’s wide range of capabilities. The user can adjust ignition timing, idle speed and rev limit, calibrate the speedometer, control decal pop, read and clear diagnostic codes and perform many other functions. TMax Auto Support has a tuning wizard that allows you to specify any symptoms your bike may be experiencing along with other significant information. Once the “send” key is hit, your data is transmitted over the Internet to ThunderMax support staff.
A TMax AutoSupport CD is included when a ThunderMax with AutoTune product is purchased. I’ve loaded it onto my laptop, and the cool thing is that rather than taking my bike to a shop to diagnose any problems, I can easily collect and upload the data to the ThunderMax support staff myself. Zipper’s is known for its outstanding customer service, and the staff readily works with their customers to resolve their performance or tuning issues.
Sussex Hills Ltd.'s Norm Gross and Matt Liptak upload data to the TMax Auto Support software
Sussex Hills Ltd.’s Norm Gross and Matt Liptak upload data to the TMax Auto Support software

The ThunderMax AutoTune has made my Switchback run much better than a brand-new motorcycle. It’s a pure delight to ride, and I’m confident that the bike can now get me out of sticky situations when I need to accelerate quickly. Rosie, I’m going to send those O2 Sensor Adapters back to Zipper’s because I won’t be needing them after all.
ThunderMax Auto Tuning Fuel Injection Performance Module for 2012-2013 Dyna—$899.95
(prices vary slightly for other Harley-Davidson models)
Zipper’s HiFlow Air Filter Kit—$129.95-$149.95
Available in cast, black and chrome for cable-operated and throttle-bywire EFI models



  1. Purchased a new 2013 CVO Roadglide from a local Harley Davidson Dealer with a ThunderMax with auto tune installed at that time. In 2015 I had HD installed 259E cams, head work and SE 10:5;1 Pistons. As well as D & D 2 into 1 Pipes. I have been working with Thunder Max over the last month and a half trying to get a Map to make this bike perform. Thundermax does not have a Map already to go for this set up. There is not a HD dealer in the state of WV that will touch this tuner. At this point I am tied of messing with this “auto tuner” and it appears that ThunderMax only goes to Daytona FL as when I met them there they said they go to no other shows or events. It is coming off and I am getting something that a HD dealer will be willing to service or has the knowledge to work on. I feel that the “Auto tune” feature is misleading as you still have to go in on a laptop and do the auto tune yourself as well as the timing adjustments. No person to person talks and everything is via email. You get an answer from someone different every time you email your updated issue. ZIppers parts as well as HD parts and no Map available not to mention support for this product..

  2. I have the 120st and the thundermax is just not doing it for me …! I get piping as I let off to throttle and as I look at the exhaust its looks very lean on the rear cyl. Also throttle is jerky as one tries to accelerate smoothie !
    Dan K.

  3. I put the Thundermax with auto tune on my 2007 Road King classic at the same time I installed a set of cobra true duals. I went in on my laptop picked a basic preprogrammed map for aftermarket air cleaner true duals and stock motor plugged in to usb on thundermax flipped ignition per instructions. My head temperature dropped 27 degrees using a laser thermometer the throttle response is insane I out accelarate three of my friends who have 2003 2005 2007 Road kings same mods air cleaner pipes different brand computers. I loved it so much my mother put it on her 2010 fatboy and at 30000 mes she says it has never run as smooth fast and cool ever and she has had it since day one. I am completely computer retarded and Reading instructions works pick a map plug usb from laptop to thundermax and in 15 minutes I was ripping down the road. Best mod ever .

  4. I bought my ThunderMax Autotune within a month of buying my 2011 CVO RoadGlide and starting changing pistons, cams, header pipes and 2 or 3 sets of different mufflers whenever the whim hit.
    I am not in any way connected to T/M but everything and every time was made a breeze by their unit. I even looked forward to making these changes because of the confidence gained. I still have my R/G and now have a new 2013 (never titled until 2016) CVO Breakout that I will soon be adding a new T/M to.
    I strongly suspect any bad rhetoric is due to lack of care and attention to the O2 sensor wiring when installing the sensors. I am a satisfied believer.

  5. T-Max is the premiere tuner used today , great product support !! NOT a Band -aid for other tuning issues or poor mechanical set up . We have clients on third units just because of additional cycle purchases !! Data logging allows us to pick out performance modifications taylored to clients riding habits the first time !! with great results and repeat work . Find a good shop to assist and work should speak for itself !!! A very big part of my shop over the years and repeat clients say it all !!!

  6. There are a lot of top line shops that can handle your issues , support has been great over the years , all their techs {Zipper’s & Thunder-Heart} are top shelf and ready to assist and my reply is from years of experiance and great results !!!


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